Here are a few ideas about adding Threema:
Threema displays an “add contact” website for every ID. Here's mine, for reference: https://threema.id/28KF76T3 However, the content can't be edited by the user.
You can add your own e-mail address to your Threema ID (you get a confirmation e-mail to verify), which allows your friends to be notified when you join Threema (if they have stored your e-mail address). Keyoxide could theoretically use this to check whether the PGP key's e-mail address belongs to the claimed ID, though this would require a verified PGP e-mail address in the first place.
I'm not aware of any other publicly available information about Threema IDs.